For many applications, a source of optical radiation of high brightness is required which can be effectively used with associated optics to provide intense illumination. In the case of the projection of a photographic image, the brightness of the primary source, and the ability of the optics to collect and direct it through the film area and into the projection lens, determine the adequacy of the projected image brightness. In search lights and beacons, the source brightness and the nature of the associated optics that are required determine how concentrated and intense the beam can be for a given size of the illuminator, i.e., a light source and its associated optics, such as the reflector, lenses and light carriers. In the case of many scientific, medical and industrial applications that each require an intense source of optical radiation, similar considerations often apply and influence the equipment cost, size and performance capability.
The most generally used source of optical radiation when high brightness is required is the short arc lamp. These have electrodes rather closely spaced in a relatively large fused silica envelope. The lamps operate with a high internal gas pressure to improve arc intensity and efficiency. The lamps have a rather low electrical impedance and require a high current. Due to this the anode dissipation, or electrical energy lost, is high being typically one third of the power input. This reduces lamp efficiency and requires the use of an oversized anode to maintain intensity and to permit adequate cooling.
The radiation from the lamp is symmetrical about the axis defined by the electrodes and is in the form of a broad distribution about the radial plane. This distribution determines the nature of the collection optics when good efficiency is required. A parabolic shaped reflector positioned about the lamp is generally used when far-field illumination is needed, as is the case with search lights and beacons. This arrangement is adequate for many purposes but it does not permit minimum divergence to be obtained due to variation in the distance of the reflector surface from the arc. Light that is incident on the reflector surface closest to the source has higher divergence. An ellipsoidal reflector is often used when efficient near-field illumination is needed, as is the case with the illumination of photographic film for image projection. In this type of reflector, the lamp is placed at one focal point and the input to the projection lens is placed at the other focal point. The collection of radiation in an ellipsoidal reflector is good but the optical aberrations prevent directing the light to the required output area with optimum concentration. Also the angular divergence of the light incident on the lens is excessive so that a relatively fast (maximum aperture) projection lens is required to utilize the light.
In addition to the above problems in providing optimum illumination, it should be mentioned that short arc lamps exhibit some instability in the position of their arc which can be very undesirable in some applications. They also require specific orientations when operating, which often imposes operational and equipment design problems. These lamps also, are somewhat dangerous to handle because of high internal pressure and sometimes explode when in operation.
A further problem that occurs with some forms of annular ring electrodes is a tendency for the arc to not distribute the current that it carries in a adequately uniform manner in its conduction to the anode. This can result in one segment of the window becoming overheated and another segment being underheated. Conceivably this condition perhaps could result in melting or thermal fracturing of the window or alternatives, in the deposition of an ode material on the window.